Quinacridones are well known compounds which are used as pigments. Quinacridones are usually prepared by oxidizing dihydroquinacridones in an alkaline medium in the presence of solvents and then dry- or wet-grinding the resulting coarsely crystalline crude pigments, or by ring closure of 2,5-dianilinoterephthalic acid in polyphosphoric acid or polyphosphoric ester and then phase-converting and finishing the resulting finely divided crude pigments with organic solvents.
Under certain conditions different quinacridones mix with each other to form solid solutions which are quite different from both physical mixtures of the compounds and from the compounds themselves. A solid solution is defined as a solid, homogeneous mixture of two or more constituents which may vary in composition between certain limits and remain homogeneous. In a solid solution, the molecules of the components enter into the same crystal lattice, usually, but not always, that of one of the components. The x-ray diffraction pattern of the resulting crystalline solid is characteristic of that solid and can be clearly differentiated from the pattern of a physical mixture of the same components in the same proportion. In such physical mixtures, the x-ray pattern of each of the components can be distinguished, and the disappearance of many of these lines is one of the criteria of the formation of solid solutions. Solid solutions are also referred to as mixed crystals.
Mixtures of quinacridone isomers are disclosed in Great Britain Application No. 1,390,093 and Helvitica Chimica Acta (1972), 55(1), 85–100.
In contrast to simple physical mixtures wherein the color is usually a direct function of the additive effects of the two or more components, solid solutions give unexpected and unpredictable hues. It is impossible to generalize about the direction or the degree of color shift.
Moreover, it is frequently observed that a remarkable enhancement of lightfastness is accompanied by the formation of solid solutions. In physical mixtures of two pigments, the components show their individual behaviors, frequently resulting in marked changes of hue as one fades more than the other. In contrast, solid solutions behave as single substances with respect to any change in hue and characteristically show superior lightfastness.